Fluorine-containing elastic copolymers, curable composition containing the same and sealant made therefrom

ABSTRACT

A fluorine-containing elastomeric copolymer comprising 55 to 62 mole % of repeating units derived from tetrafluoroethylene and 38 to 45 mole % of repeating units derived from perfluoro(methyl vinyl ether) which is obtained by radically polymerizing the monomers in the presence of a diiodide compound of the formula: RI 2  in which R is a saturated fluorohydrocarbon or chlorofluorohydrocarbon group having 1 to 16 carbon atoms, or a hydrocarbon group having 1 to 3 carbon atoms and which has a Mooney viscosity (ML 1+10  100° C.) in the range between 20 and 150. This copolymer has an excellent compression set at high temperature, which is an important property for sealing members.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/JP96/03732 which has an Internationalfiling date of Dec. 20, 1996 which designated the United States ofAmerica, the entire contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a fluorine-containing elastomericcopolymer, a curable composition comprising the same, and a sealingmember prepared from the same. In particular, the present inventionrelates to a fluorine-containing elastomeric copolymer which is preparedby polymerizing monomers in the presence of a specific diiodidecompound, a curable composition comprising the same, and a sealingmember prepared from the same.

PRIOR ART

Fluorine-containing elastomeric copolymers are used in a wide variety offields such as equipment for producing semiconductors, chemical plants,drilling rigs, space hardware, and the like, because of their excellentchemical resistance, heat resistance and compression set.

In particular, sealing members comprising the fluorine-containingelastomeric copolymers have the large merit of the reduction of runningcosts due to the maintenance-free. Thus, fluorine-containing elastomericcopolymers having a longer life and higher reliability are sought.

Fluorine-containing elastomeric copolymers having iodine atoms in apolymer chain, which can be cured with peroxides, can be easilyvulcanized without the addition of contaminant additives such as metaloxides, and thus they are used as materials of clean sealing memberswhich are used in the equipment for producing semiconductors. However, acopolymer of tetrafluoroethylene and a perfluorovinyl ether having apolyether-type perfluoroalkyl group has insufficient dry strength, andalso inferior compression set at high temperature to commerciallyavailable cured materials of fluororubbers comprising vinylidenefluoride, as described in JP-A-62-89713 and JP-A-62-12734.

Conventional copolymers of tetrafluoroethylene-perfluoro(methyl vinylether) having iodine atoms, which are disclosed in JP-A-4-505345, haveinsufficient tensile properties and compression set.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a fluorine-containingelastomeric copolymer which can provide a clean sealing member having along life and high reliability as desired particularly in thesemiconductor field.

As the result of extensive study for achieving such an object, it hasbeen found that a fluorine-containing elastomeric copolymer comprisingtetrafluoroethylene and perfluoro (methyl vinyl ether) in a specificratio and containing an iodine atom has good dry properties, and itscompression set at high temperature, which is one of the most importantproperties for sealing members, is better than that of a polyol-curablefluororubber comprising vinylidene fluoride and hexafluoropropylene,which has the best compression set among commercially available rubbers.

Furthermore, it has been found that such a fluorine-containingelastomeric copolymer, which is obtained by polymerizingtetrafluoroethylene and perfluoro(methyl vinyl ether) at a specifictemperature (between 40 and 60° C.), has a further improved compressionset.

According to the first aspect, the present invention provides afluorine-containing elastomeric copolymer comprising 55 to 62 mole % ofrepeating units derived from tetrafluoroethylene (TFE) and 38 to 45 mole% of repeating units derived from perfluoro(methyl vinyl ether) (PMVE)which is obtained by radically polymerizing the monomers in the presenceof a diiodide compound of the formula:

    RI.sub.2                                                   (1)

wherein R is a saturated fluorohydrocarbon or chlorofluorohydrocarbongroup having 1 to 16 carbon atoms, or a hydrocarbon group having 1 to 3carbon atoms, preferably at a temperature in the range between 40 and60° C., and which has a Mooney viscosity (ML₁₊₁₀ 100° C.) in the rangebetween 20 and 150.

According to the second aspect, the present invention provides afluorine-containing elastomeric copolymer comprising 50 to 80 mole % ofrepeating units derived from tetrafluoroethylene, 20 to 50 mole % ofrepeating units derived from perfluoro (methyl vinyl ether), and 0.01 to1 mole %, based on the total mole of tetrafluoroethylene andperfluoro(methyl vinyl ether), of repeating units derived from aniodine-containing fluorinated vinyl ether of the formula:

    I(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.m --[CF(CF.sub.3)CF.sub.2 O].sub.n --CF═CF.sub.2                                         (2)

wherein m is an integer of 1 to 5, and n is an integer of 0 to 3, whichis obtained by radically polymerizing the monomers in the presence of adiiodide compound of the above formula (1), and which has a Mooneyviscosity (ML₁₊₁₀ 100° C.) in the range between 20 and 150.

According to the third aspect, the present invention provides afluorine-containing elastomeric copolymer comprising 55 to 62 mole % ofrepeating units derived from tetrafluoroethylene, 38 to 45 mole % ofrepeating units derived from perfluoro(methyl vinyl ether), and 0.01 to1 mole %, based on the total mole of tetrafluoroethylene andperfluoro(methyl vinyl ether), of repeating units derived from aniodine-containing fluorinated vinyl ether of the above formula (2),which is obtained by radically polymerizing the monomers in the presenceof a diiodide compound of the above formula (1), and which has theMooney viscosity (ML₁₊₁₀ 100° C.) in the range between 20 and 150.

According to the fourth aspect, the present invention provides a curablefluorine-containing elastomeric copolymer composition comprising 100 wt.parts of the above fluorine-containing elastomeric copolymer, 0.05 to 10wt. parts of an organic peroxide, and 0.1 to 10 wt. parts of acrosslinking aid.

According to the fifth aspect, the present invention provides a sealingmember prepared from the above composition.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a DSC(differential scanning calorimeter) chart for a polymerobtained in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Conventional TFE/PMVE copolymers have a composition of 65-70 mole % ofTFE and 30-35 mole % of PMVE in view of copolymerizability and costs, sothat they exhibit rubbery properties. The polymerization has beencarried out at a temperature in the range between 70 and 90° C. in thepresence of ammonium persulfate.

In the case of fluorine-containing elastomeric copolymers havingterminal iodine atoms, it is known that the polymer composition andpolymerization temperature have unexpectedly large influences on theproperties of the copolymers. However, the TFE/PMVE copolymers of thepresent invention comprising 55 to 62 mole % of TFE and 38 to 45 mole %of PMVE have very good compression set at high temperature which cannotbe achieved by the copolymers which have the conventional monomercomposition and are prepared at the conventional polymerizationtemperature. In addition, the copolymers of the present invention havegood compression set at room temperature in spite of their relativelyhigh Tg of about -4° C. Furthermore, the copolymers of the presentinvention which are prepared by carrying out the polymerization at atemperature in the range between 40 and 60° C. have further improvedcompression set at high and room temperatures.

This means that the polymer chains of the copolymer of the presentinvention with the above limited monomer composition have much higherrepulsion force and recovery force than those having other monomercompositions. In addition, the polymerization temperature significantlyincreases the crosslinking efficiency of the cured material of thecopolymer.

When the ratio of PMVE is less than 38 mole %, the compression setdeteriorates. When the ratio of PMVE exceeds 45 mole %, thecopolymerization rate decreases greatly, and thus the productivity ofthe copolymer decreases.

The fluorine-containing elastomeric copolymer of the present inventionis obtained by radically copolymerizing TFE and PMVE in the presence ofa diiodide compound of the above formula (1), and the amount of iodineatoms which are introduced in the copolymer from the diiodide compoundof the formula (1) should be in the range between 0.01 and 1 wt. % basedon the whole weight of the copolymer. The amount of the iodine atoms iscalculated from the weight of the diiodide compound which is addedduring the polymerization. The amount of the diiodide compound which ispresent in the copolymer has a great influence on the determination of amolecular weight, since the diiodide compound acts as a chain transferagent as described in JP-A-53-125491, and further the iodine atomsintroduced in the polymer functions as crosslinking sites. Therefore,when the amount of the iodine atoms which are introduced in thecopolymer from the diiodide compound of the formula (1) is less than0.01 wt. % based on the whole weight of the copolymer, the molecularweight of the polymer becomes too larger, and thus the polymer loses theflowability in the curing reaction, or a crosslinking density decreases.Thus, no molded article can be obtained, or only insufficient sealingproperties are achieved. When the amount of the iodine atoms exceeds 1wt. %, the molecular weight becomes too low, and thus some troubles mayarise in the kneading and molding steps.

The copolymer of the present invention may comprise 0.01 to 1 mole %,based on the total amount of TFE and PMVE, of an iodine-containingfluorinated vinyl ether of the formula:

    I(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.m --[CF(CF.sub.3)CF.sub.2 O].sub.n --CF═CF.sub.2                                         (2)

wherein m is an integer of 1 to 5, and n is an integer of 0 to 3, inaddition to TFE and PMVE.

Preferable examples of the iodine-containing fluorinated vinyl ether ofthe formula (2) are

ICH₂ CF₂ CF₂ OCF═CF₂,

I(CH₂ CF₂ CF₂ O)₂ CF═CF₂,

I(CH₂ CF₂ CF₂ O)₃ CF═CF₂,

ICH₂ CF₂ CF₂ OCF(CF₃)CF₂ OCF═CF₂,

ICH₂ CF₂ CF₂ O[CF(CF₃)CF₂ O]₂ CF═CF₂, and the like.

Among them, ICH₂ CF₂ CF₂ OCF═CF₂ is more preferable.

The terpolymer of TFE/PMVE/iodine-containing fluorinated vinyl ether isa novel copolymer, and it preferably comprises 55 to 62 mole % of TFErepeating units, 38 to 45 mole % of PMVE repeating units, and the amountof the iodine-containing fluorinated vinyl ether is between 0.01 and 1mole % based on the total amount of TFE and PMVE.

However, the copolymer has certain effects even when the amounts of TFErepeating units and PMVE repeating units are between 50 and 80 mole %and between 20 and 50 mole %, respectively, and the amount of theiodine-containing fluorinated vinyl ether is between 0.01 and 1 mole %based on the total amount of TFE and PMVE. That is, the copolymercontaining the iodine-containing fluorinated vinyl ether of the formula(2) in addition to TFE and PMVE has a compression set to a certaindegree, when the amount of PMVE is at least 20 mole %, and thepolymerization rate does not decrease significantly up to 50 mole % ofPMVE.

The fluorine-containing elastomeric copolymer of the present inventionhas a Mooney viscosity (ML₁₊₁₀ 100° C.) of between 20 and 150. TheMooney viscosity herein used means a viscosity measured by the methoddefined in JIS K 6300 "Mooney viscosity measurement" at a measuringtemperature of 100° C.

When the Mooney viscosity is less than 20, some trouble may arise duringthe kneading step. When the Mooney viscosity exceeds 150, the copolymerloses the flowability in the curing reaction.

The copolymer of the present invention may be prepared by the methoddescribed in Examples 12-15 of JP-A-62-12734 (=EP-A-199138) withmodifying the kinds and amounts of monomers.

That is, TFE, PMVE and optionally other copolymerizable monomer such asHFP are radically emulsion polymerized in an aqueous medium underpressure while stirring, in the presence of a diiodide compound and inthe presence of substantially no oxygen.

Typical examples of the diiodide compound which is used in thepreparation of the copolymer of the present invention are1,3-diiodoperfluoropropane, 1,4-diiodoperfluorobutane,1,3-diiodo-2-chloroperfluoropropane,1,5-diiodo-2,4-dichloroperfluoropentane, 1,6-diiodoperfluorohexane,1,8-diiodoperfluorooctane, 1,12-diiodoperfluorododecane,1,16-diiodoperfluorohexadecane, diiodomethane, and 1,2-diiodoethane.These compounds may be used singly or in combination with each other.Among them, 1,4-diiodoperfluorobutane is preferable.

The amount of the diiodide compound is between 0.01 and 1 wt. % based onthe whole weight of the copolymer.

It is possible to copolymerize a monomer having an iodine atom with thefluorine-containing elastomeric copolymer of the present invention. Thecopolymerization of a monomer having an iodine atom can further increasethe compression set of the copolymer. Perfluorovinyl ether compounds arepreferable as monomers having an iodine atom, in view of theircompolymerizability. For example,perfluoro(6,6-dihydro-6-iodo-3-oxa-1-hexene) andperfluoro(5-iodo-3-oxa-1-pentene), which are disclosed in JP-B-5-63482and JP-A-62-12734, are preferable.

When the polymerization temperature exceeds 60° C., the compression settends to deteriorate although the dry properties may not be influenced.When the polymerization temperature is less than 40° C., thepolymerization rate is low if a persulfate is used alone. In addition,the polymerization rate is still low, even when a redox systemcontaining a sulfite salt, etc. is used, and furthermore, metal ions ofreducing agents remain in the polymer. The polymer containing metal ionscannot be used in the field of semiconductor production.

Radical polymerization initiators which are used for the preparation ofthe copolymer of the present invention may be the same as those used inthe conventional polymerization processes for fluorine-containingelastomers. Such initiators include organic and inorganic peroxides, andazo compounds. Specific examples of the initiator include persulfates,peroxycarbonates, peracid esters, and the like. A preferable initiatoris ammonium persulfate (APS). APS may be used singly or in combinationwith a reducing agent such as a sulfite.

It is preferable to avoid the use of reducing agents which are thesources of metal ions, since the prepared copolymers are often used assealing members, which are employed in an equipment for producingsemiconductors which is required to have cleanness.

Emulsifiers, which are used in the emulsion polymerization for thepreparation of the copolymer of the present invention, ay be selectedfrom a wide variety of emulsifiers. Salts of carboxylic acids having afluorocabon or fluoropolyether chain are preferable in view of thesuppression of a chain transfer reaction onto emulsifier moleculesduring the polymerization. The amount of the emulsifier is preferably inthe range between about 0.05 and 2 wt. %, in particular in the rangebetween 0.2 and 1.5 wt. %, based on water added.

The monomer mixture gas used in the pre sent invention is explosive asdescribed by G. H. Kalb et al, Advances in Chemistry Series, 129,13(1973). Thus, it is necessary to devise a polymerization reactor sothat no spark, that is an ignition source, will occur. In that sense,the polymerization pressure is preferably reduced as low as possible.

The polymerization pressure can be changed in a wide range. In general,the polymerization pressure is in the range between 0.5 and 5 MPa.Preferably, the polymerization pressure is at least 0.8 MPa in view ofthe productivity, since the polymerization rate increases as thepolymerization pressure increases.

The copolymer of the present invention is crosslinked and cured(vulcanized) with various crosslinking sources to provide afluororubber. The crosslinking sources are preferably organic peroxides,while high energy electromagnetic waves such as radiation (e.g. α-rays,β-rays, γ-rays, electron beams , X-rays, etc.), UV-light, etc. may beused.

The amount of the organic peroxide is in the range between 0.05 and 10wt. parts, preferably in the range between 1.0 and 5 wt. parts, per 100wt. parts of the copolymer.

The organic peroxides are preferably peroxides which easily generateperoxy radicals by the application of heat or in the presence of redoxsystems. Examples of the organic peroxides include1,1-bis(tert.-butylperoxy)-3,5,5-trimethylcyclohexane,2,5-dimethylhexane-2,5-dihydroxyperoxide, di-tert.-butylperoxide,tert.-butylcumylperoxide, dicumylperoxide,α,α-bis(tert.-butylperoxy)-p-diisopropylbenzene,2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane,2,5-dimethyl-2,5-di(tert.-butylperoxy)-hexyne-3, benzoylperoxide,tert.-butylperoxybenzene, 2,5-dimethyl-2,5-di(benzoiylperoxy)hexane,tert.-butyl peroxymaleate, tert.-butylperoxyisopropyl carbonate, and thelike. Among them, diallyl type ones are preferable. In particular,2,5-dimethyl-2,5-di(tert.-butylperoxy)hexane is preferable.

The kinds and amounts of the organic peroxides are selected by takinginto account the amount of active --O--O-- groups, decompositiontemperatures, etc.

When the organic peroxides are used, the curing is enhanced by thecombined use of crosslinking aids. Any crosslinking aids may be used, inprinciple, insofar as they have a reactivity with peroxy radicals andpolymer radicals, and the kinds of the crosslinking aids are notlimited. Preferable examples of the crosslinking aids include triallylcyanurate, triallyl isocyanurate, trially formal, triallyl trimellitate,N,N'-m-phenylenebismaleimide, dipropargyl terephthalate, diallylphthalate, tetraallyl terephthalic amide, triallyl phosphate, and thelike. Among them, triallyl isocyanurate is particularly preferable.

The amount of the crosslinking aid is preferably in the range between0.1 and 10 wt. parts, more preferably between 0.5 and 5 wt. parts, per100 wt. parts of the copolymer.

The fluororubber of the present invention may be co-crosslinked withother rubber or material. Examples of the other rubber or material,which can be blended and co-crosslinked with the fluororubber of thepresent invention, are silicone oils, silicone rubbers, ethylene-vinylacetate copolymers, 1,2-polybutadiene, fluorosilicone oils,fluorosilicone rubbers, fluorophosphazene rubber,hexafluoropropylene-ethylene copolymers, tetrafluoroethyene-propylenecopolymers, other polymers having a radical reactivity. The amount ofthe other rubber or material is not limited, but should not be so largeas to deteriorate the inherent properties of the copolymer of thepresent invention.

Furthermore, the copolymers of the present invention may containpigments for coloring the copolymer, fillers, reinforcing materials, andthe like. Examples of the generally used fillers or reinforcingmaterials are inorganic ones such as carbon black, TiO₂, SiO₂, clay,talc, etc., and organic ones such as fluorine-containing polymers, forexample, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylfluoride, polychlorotrifluoroethylene, tetrafluoroethylene-ethylenecopolymers, tetrafluoroethylene-vinylidene fluoride copolymers, and thelike.

Mixing means for these curing components are suitably selected accordingto the viscoelasticities and states of the components. In general, openroll, powder mixers, and the like are used. Alternatively, solidcomponents are dissolved or dispersed in solvents, and then mixed.

The curing temperature and time depend on the kind of the peroxide used.In general, the press curing is carried out at a temperature in therange between 150 and 200° C. for 3 to 30 minutes, and the oven curingis carried out at a temperature in the range between 150 and 250° C. for1 to 24 hours.

The copolymers of the present invention are advantageously used asgeneral molding materials, sealants, adhesives, paints, and the like.Preferable applications of the molded articles are sealing members, suchas O-rings, lip-type packings, oil seals, diaphragms, gaskets, V-rings,and the like.

EXAMPLES

The present inventions will be illustrated by the following examples.

Example 1

Pure water (2 liters), C₇ F₁₅ COONH₄ (20 g) as an emulsifier anddisodium hydrogen phosphate dodecahydrate (0.18 g) as a pH-regulatorwere charged in a SUS 316 autoclave (internal volume of 6 liter) whichhad no ignition source. After thoroughly replacing the internalatmosphere with nitrogen gas, the content was heated to 50° C. whilestirring at 600 rpm, and then, TFE and PMVE in a molar ratio of 24:76were injected under pressure so that the internal pressure rose to 12.0kgf/cm² G. After that, the aqueous solution of ammonium persulfate (APS)in a concentration of 186 mg/ml (2 ml) was injected under pressure ofnitrogen gas to initiate a polymerization reaction.

The internal pressure dropped as the polymerization reaction proceeded.When the pressure dropped to 11.0 kgf/cm² G, a diiodide compound,I(CF₂)₄ I (4.0 g) was injected under pressure. Then, 20.0 g of TFE wasinjected under its own pressure and 22.0 g of PMVE was injected with aplunger pump, and thus the increase and drop of the pressure wererepeated.

After 8.4 hours from the start of the polymerization reaction, the totalamount of TFE and PMVE reached 860 g. Then, the autoclave was cooled,and the unreacted monomers were discharged. An aqueous emulsion having asolid content of 30 wt. % was obtained.

The aqueous emulsion was poured in a beaker and frozen in adry-ice/methanol bath to coagulate the polymer. After defreezing, thecoagulated polymer was washed with water and dried under reducedpressure. Thus, a rubbery polymer (862 g) was obtained. The Mooneyviscosity ML₁₊₁₀ 100° C. was 60.

The results of the ¹⁹ F-NMR analysis indicated that the polymer had themonomeric unit composition of 61.7 mole % of TFE and 38.3 mole % ofPMVE. The content of iodine atoms, which was calculated from theelemental analysis, was 0.18 wt. %. The glass transition temperature Tg(median) of the polymer, which was measured with a differential scanningcalorimeter (DSC), was -3° C. (see the DSC chart in FIG. 1).

Example 2

A polymerization reaction was carried out in the same manner as inExample 1 except that the initial molar ratio of the monomers and theamounts of the monomers, which were charged to increase the pressure,were changed as listed in Table 1, a diiodide compound ICH₂ CF₂ CF₂OCF═CF₂ (each 1.5 g) was injected when the total charged amount of TFEand PMVE reached 430 g, 511 g, 596 g and 697 g, and the aqueous solutionof APS in a concentration of 35 mg/ml (each 2 ml) was injected underpressure of nitrogen gas every 12 hours after the start of thepolymerization reaction to continue the polymerization reaction. After35 hours, the polymerization reaction was terminated.

The aqueous emulsion was coagulated, washed and dried in the same way asin Example 1, and thus the rubbery polymer (872 g) was obtained. TheMoony viscosity (ML₁₊₁₀ 100° C.) of the polymer was 63. The results ofthe ¹⁹ F-NMR analysis indicated that the polymer had the monomeric unitcomposition of 59.2 mole % of TFE and 40.8 mole % of PMVE. The contentof iodine atoms was 0.30 wt. %.

Comparative Example 1

A polymerization reaction was carried out in the same manner as inExample 2 except that the initial molar ratio of the TFE/PMVE monomermixture was changed to 27:73, and the amounts of the monomers, whichwere additionally charged, were changed as listed in Table 1. After 29hours, the polymerization reaction was terminated.

The aqueous emulsion was coagulated, washed and dried in the same way asin Example 1, and thus the rubbery polymer (847 g) was obtained. TheMooney viscosity (ML₁₊₁₀ 100° C.) of the polymer was 58. The results ofthe ¹⁹ F-NMR analysis indicated that the polymer had the monomeric unitcomposition of 62.9 mole % of TFE and 37.1 mole % of PMVE. The contentof iodine atoms was 0.28 wt. %.

Comparative Example 2

A polymerization reaction was carried out in the same manner as inExample 2 except that the polymerization temperature was changed to 80°C., the initial molar ratio of the TFE/PMVE monomer mixture was changedto 29:71, and the concentration of the aqueous APS solution was changedto 20 mg/ml. After 45 hours, the polymerization reaction was terminated.

The aqueous emulsion was coagulated, washed and dried in the same way asin Example 1, and thus the rubbery polymer (850 g) was obtained. TheMooney viscosity (ML₁₊₁₀ 100° C.) of the polymer was 55. The results ofthe ¹⁹ F-NMR analysis indicated that the polymer had the monomeric unitcomposition of 64.0 mole % of TFE and 36.0 mole % of PMVE. The contentof iodine atoms was 0.34 wt. %.

The components shown in Table 1 were compounded in each of thecopolymers obtained in Examples, and a curable composition was prepared.Then, curing properties were measured with a curastometer (JIS I type)at 160° C.

The composition was press cured at 160° C. for 10 minutes and then ovencured at 180° C. for 4 hours, and the properties of the obtained curedmaterial were measured. The properties of the cured material weremeasured according to JIS K 6301. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                   Example 1                                                                            Example 2                                                                              Example 3                                                                              Example 4                                 ______________________________________                                        Polymerization conditions                                                       Temperature (° C.)                                                                    50       50     50     80                                      I(CF.sub.2 CF.sub.2).sub.2 I (g) 4.0 4.0 4.0 4.0                              ICH.sub.2 CF.sub.2 CF.sub.2 OCF═CF.sub.2 --  6.0 6.0 6.0                  (g)                                                                           Initial molar ratio 24:76 20:80 27:73 29:71                                   of monomers                                                                   (TFE:PMVE)                                                                    Amounts of supplemental 20/22 19/23 21/21 22/20                               monomers (g)                                                                  (TFE/PMVE)                                                                    Polymerization time (hrs) 8.4 35 29 45                                        Polymer amount 862 872 847 850                                                Properties of uncured                                                         copolymer                                                                     Iodine content (wt. %) 0.18 0.30 0.28 0.34                                    Polymer composition 61.7:38.3 59.2:40.8 62.9:37.1 64.0:36.0                   (TFE:PMVE)                                                                    Mooney viscosity 60 63 58 55                                                  ML.sub.1+10 100° C.                                                    Tg (° C.) (DSC -3 -5 -3 -3                                             measurement)                                                                Curing properties (160° C.)                                              Minimum torque (kg)                                                                          0.09     0.10   0.09   0.12                                    Maximum torque (kg) 5.13 5.41 5.24 4.60                                       Induction time (min.) 0.6 0.7 0.8 0.8                                         Optimum curing time 1.8 1.6 2.3 2.3                                           (min.)                                                                        Dry properties                                                                100% modulus (MPa) 11.7 11.2 12.6 11.3                                        Tensile strength (MPa) 21.5 19.7 20.2 17.8                                    Elongation at break (%) 160 140 130 150                                       Hardness (JIS A) 84 82 82 83                                                  Compression set (%)                                                           (P240 O-ring)                                                                 Room temp. × 72 hours 9.7 6.4 12.4 10.2                                 200° C. × 72 hours 14.7 12.2 15.2 18.6                           200° C. × 168 hours 22.4 19.7 24.4 28.6                        ______________________________________                                         Curing composition: 100 wt. parts of a copolymer, 8 wt. parts of MT           thermal carbon, 7 wt. parts of SRF carbon, 3 wt. parts of triallyl            isocyanurate (TAIC), and 1 wt. part of Perhexa2, 5B.                     

What is claimed is:
 1. A fluorine-containing elastomeric copolymercomprising:(a) 55 to 62 mole % of repeating units obtained fromtetrafluoroethylene monomer; and (b) 38 to 45 mole % of repeating unitsobtained from perfluoro(methyl vinyl ether) monomer; andwherein saidcopolymer is obtained by radically polymerizing said monomers in thepresence of a diiodide compound of the formula

    RI.sub.2,

wherein R is a saturated fluorohydrocarbon or chlorofluorohydrocarbongroup having 1 to 16 carbon atoms, or R is a hydrocarbon group having 1to 3 carbon atoms; and wherein said copolymer has a Mooney viscosity(ML₁₊₁₀ 100° C.) in the range between 20 and
 150. 2. A copolymer as inclaim 1, wherein said polymerizing takes place at a temperature between40 and 60° C.
 3. A copolymer as in claim 1, further comprising:0.01 to 1mole %, based on the total moles of said monomers, of repeating unitsobtained from an iodine-containing fluorinated vinyl ether monomer ofthe formula:

    I(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.m --[CF(CF.sub.3)CF.sub.2 O].sub.n --CF═CF.sub.2

wherein m is an integer of 1 to 5, and n is an integer of 0 to
 3. 4. Acurable fluorine-containing elastomeric copolymer composition comprising100 wt. parts of a fluorine-containing elastomeric copolymer as claimedin any one of claims 1 or 2, 0.05 to 10 wt. parts of an organicperoxide, and 0.1 to 10 wt. parts of a crosslinking aid.
 5. A sealingmember prepared from a composition as claimed in claim 4.